During the course of surface processing or surface cleaning of silicon wafer in the photovoltaic manufacturing application, a mixture containing mainly hydrofluoric acid (HF) and nitric acid (HNO3) is used. For example, in a process of surface texturing for crystalline silicon solar cell, such acid mixture solution is used. However, when a fluoro-nitric acid mixture is applied as the treating solution, silicon impurities, such as fluorosilicic acid (hexafluorosilicic acid or hydrofluorosilicic acid; H2SiF6), remain in the solution after etching the surface of silicon wafer. The spent solution containing silicon impurities has less etching performance, and thus, the spent solution is sent to waste when the silicon concentration reaches a critical level and a new solution is introduced for further etching process (so called “batch process”).
The disposal of the spent HF/HNO3 mixture solution containing high level of silicon impurity is difficult in an industrial size due to high load of fluorine and nitrogen. Also, such disposal leads to high amount of consumption of the mixture solution of hydrofluoric acid and nitric acid, which is not economically favored. As such, an efficient purification and recycling of the spent HF/HNO3 mixture solution containing silicon impurity could be advantageous.
Japanese patent Kokai No. 2012-125723 discloses an apparatus for removing hydrofluorosilicic acid contained in the waste liquid of a nitrohydrofluoric acid, and recovering the refined liquid of the nitrohydrofluoric acid by an electrodialysis having a specific configuration. However, the electrodialysis described in said patent application requires continuous use of further reagents, which may cause another economically unfavorable issue, and in addition, the reduction degree of silicon impurity by the above-mentioned method was not sufficient. For example, the concentration of H2SiF6 in feed solution which is 2% is reduced to 0.5% at outlet of the purification step in Example 1 of the Japanese patent application.
The invention now makes available an efficient method of purifying the solution which does not show the above-mentioned problems.